This invention relates to a liquid crystal display and, more particularly, to a method and an apparatus for driving a passive liquid crystal display.
Large passive liquid crystal displays suffer from serious contrast problems due to the manner in which the display elements are actuated. A typical passive liquid crystal display 10 is shown in FIG. 1 in simplified schematic. A display panel 12 has a plurality of row electrodes or lines 14 extending perpendicular to a plurality of column electrodes or lines 16. The crossing points of the lines 14 and 16 define pixels 18 for displaying visual information. For illustration purposes, the display is shown as having four row lines 14, R1 through R4, and five column lines 16, C1 through C5. In practice there are often hundreds of rows and columns.
Voltages are applied to the column wires and the row wires, and a pixel is addressed when both its column and row are carrying a voltage. Liquid crystal display devices typically use what is generally referred to as "a line at a time" addressing method, which is depicted by the wave forms shown in FIG. 2. All of the column lines 16 are activated simultaneously by the application of a column voltage wave form V.sub.C (t.sub.K) having a plurality of pulses. The row lines 14 are each turned on for a fixed period of time in sequence. Specifically, at a time t.sub.1, a first row voltage wave form V.sub.R1 (t.sub.K) includes a pulse which is applied to the row line R1 while an associated column voltage pulse is applied to the column lines C1 through C5 such that all of the pixels 18 in the first row of the display panel 12 are activated. At a time t.sub.2, after the first row voltage pulse has been terminated, a second row voltage wave form V.sub.R2 (t.sub.K) applies a pulse to the row line R2, while an associated column voltage pulse is applied to column lines C1 through C5 such that the second row of the pixels 18 in the display panel 12 is activated. Similarly, row voltage wave forms V.sub.R3 (t.sub.K) and V.sub.R4 (t.sub.K) apply pulses to the row lines R3 and R4 respectively; at the times t.sub.3 and t.sub.4, respectively, to activate the pixels 18 in the third and fourth rows, respectively. Once activated, the pixels remain at the same state for a limited period of time (referred to as the decay time).
In a large passive display, the pixels 18 located in the upper rows may have decayed entirely before the lower rows are addressed, such that the image in those upper row portions of the display will fade. This results in a low contrast ratio. This problem is partially solved by using liquid crystal materials whose states relax slowly in time. The problem assumes critical importance when the device is run in a video mode, where short response times are required, in direct contradiction to using slow response liquid crystals.
One can mitigate this problem with video mode operation by exciting some or all of the rows of the display panel simultaneously. The formulas typically used to generate the specified row and column voltages require a number of coefficients, with a particular coefficient being assigned to each pixel. These coefficients must be determined, and the challenge of determining all of the coefficients across all of the rows has limited the prior art ability to excite more than one row at a time for a given image. The prior art has attempted to overcome this problem to excite some or all of the rows by presetting the coefficients. However, for images that have more than one bit of gray scale, this has resulted in a coupling between the different pixels in a given column. This coupling manifests itself such that the required rms (root mean square) voltage to achieve a particular light transmission intensity in a given pixel depends on the state of all of the other pixels in that column. Decoupling the various pixels within a column from one another requires the use of a virtual row.